1. Field of the Invention
The present invention relates to an optical module, in which light emitted from a laser diode may be coupled with a transmission optical fiber via a coupling member having an elliptic outer shape.
2. Related Prior Art
Recently, as the optical communication progresses, it is requested that the optical coupling efficiency between semiconductor optical devices such as laser diode or photodiode and an optical fiber should be enhanced without complex means. One method for the optical coupling between the devices and the optical fiber is known as an active alignment technique. The Japanese patent application published as H10-096839 has disclosed such active alignment technique, in which a package including the laser diode therein and a lens aligned to the laser diode is assembled with a ferrule provided in an end portion of the optical fiber such that the optical output detected via the optical fiber becomes the maximum.
A passive alignment technique, one of the other alignment method, applied to an optical module is also well known, in which the alignment mark and the V-groove are formed on a substrate, and the devices may be coupled with the optical fiber secured within the V-groove by aligning them by using thus formed alignment mark. The Japanese patent application published as H11-190811 has disclosed the optical module including a silicon substrate, on which the V-groove and the alignment mark are formed in a same time, an optical fiber secured within the V-groove, and a semiconductor device passively aligned with the optical fiber in the V-groove.
The Japanese patent application published as 2002-328204 has disclosed an optical module assembled by another passive alignment technique, which is shown in FIG. 8. The optical module 1 shown in FIG. 8 includes a substrate 2 with a V-groove 3 provided thereon, a laser diode 4, an optical fiber and two micro lenses 6a and 6b. The micro lens has a lens 7, an arm portion 8, and a lens holder 9.
One of the micro lens 6a converts divergent light emitted from the laser diode 2 into a parallel beam, while the other micro lens 6b focuses the thus converted parallel beam on the optical fiber 5. These micro lenses 6a and 6b are made by a semiconductor substrate made of silicon single crystal, and have a convex lens 7 with a diameter thereof smaller than that of the optical fiber 5. A semiconductor process such as etching using a photolithography technique may form the convex lens 7. The arm portion 8, extending to both sides of the convex lens 7, is accompanied to pick up the lens 7 at the assembling of the module. Further, the lens holder 9 surrounds and secures the convex lens 7. These micro lenses 6a and 6b are optically aligned with the laser diode 4 and the optical fiber by positioning the lens holder 9 within the V-groove 3 as touching the outer surface of the lens holder 9 to side surfaces of the V-groove 3.
The active alignment technique is useful for an optical fiber with a core diameter of about 9 μm to optically couple in precise with the semiconductor device, such as laser diode or photodiode, or a planar waveguide. However, not only it takes a time to obtain the optimum coupling relation but also it is required for costly equipment and a machine to precisely align devices. Further, additional process such as gluing or welding is necessary for securing and fixing the devices. Accordingly, the module applying the active alignment technique has generally become a cost-effective product.
On the other hand, the passive alignment technique needs relatively short time to assemble the devices compared to the active alignment technique, because the former does not monitor the practical optical alignment between devices. However, to obtain a predetermined coupling efficiency requires a positional accuracy of devices more precise than 1 μm, thereby inevitably requesting the costly equipment. Further, when the optically sensitive spot is scattered in every laser diode, the optical coupling efficiency with the optical fiber secured in the V-groove is inevitably scattered. Thus, only the passive alignment technique can not realize the precise optical alignment between devices, and additional adjustment must be accompanied, which causes the costly optical module.
Moreover, in the case shown in FIG. 8, when the diameter of the lens holder 9 of the micro lens is smaller than that of the optical fiber, the position of the micro lens is rigidly defined in the vertical direction. On the other hand for the horizontal direction, since the outer surface of the lens holder 9 is apart from the side surface of the V-grove 3, the horizontal position of the device is not fixed. By setting the outer diameter of the lens holder 9 to be about the diameter of the optical fiber 5, the lens may be passively coupled with the optical fiber. However, in this case, one of the arm portions 8 is apart from the primary surface of the substrate and the micro lens 6 becomes mechanically unstable in their positions.
In the passive alignment, in order to horizontally align the laser diode 4 mounted on the substrate 2 with the optical fiber 5 secured in the V-groove 3, the laser diode must be finely adjusted in its horizontally position, because the optical fiber 5 and the micro lens 6 are rigidly fixed in the V-groove 3. An image recognition method may be applicable to align the laser diode with the substrate, but such apparatus is costly equipment in general, thereby leaving the subject of the machining cost, the assembling accuracy and the assembling time.